It has been estimated that somewhere between 50 and 70 million people suffer from chronic back pain in the United States. In most cases, conservative therapies, including, for example, bed rest and physical therapy will succeed in alleviating, or at least significantly reducing the back pain. Still, a significant number of patients are unaided by conservative therapies alone and undergo spinal surgery before finding relief. The rate at which caregivers and patients opt for surgery also continues to grow as medical technology advances and surgical options increase. In all, approximately 750,000 spine surgeries are performed per year in the United States alone.
When necessary, spine surgery may provide great benefit to the patient, often allowing patients to resume activities long since abandoned because of the debilitating pain. Spine surgery, however, is not without risk. Operating on or near the spine generally means operating in close proximity to delicate neural tissue, such as the spinal cord and nerve roots. Damage to the neural tissue, which may be caused, for example, by inadvertent contact with a surgical instrument and/or implant or by excessive nerve retraction, may have consequences ranging from a slight loss of sensation to complete paralysis. One way to mitigate this risk is to conduct neurophysiologic monitoring during the procedure and/or recovery period. Neurophysiologic monitoring generally consists of stimulating neural tissue and analyzing responses (generally electrical waveforms) generated by the stimulus. Processing of the electrical waveform data is generally complex. The presence of electrical energy caused by sources other than the patients neuromuscular system can taint the analysis and produce unwanted results. It is thus beneficial to be able to detect such noise and ignore or filter it out of the neurophysiologic data.